Unsaturated carboxylic acids, such as acrylic acid and methacrylic acid, are industrially important as starting materials for various synthetic resins, coating materials and plasticizers. Two-step vapor phase reaction processes from alkenes have historically been practiced for the production of unsaturated carboxylic acids, including acrylic acid and methacrylic acid, and these processes are still widely used today. However, in view of the price difference between alkanes and their corresponding alkenes (for example, propane versus propene, and isobutane versus isobutene), processes involving single-step vapor phase catalytic oxidation of alkanes, alkenes, and mixtures thereof, to produce unsaturated carboxylic acids have been developed with varying degrees of success. Generally, such methods involve subjecting an alkane, an alkene, or a mixture thereof, to a vapor phase catalytic oxidation reaction in the presence of a suitable mixed metal oxide catalyst, to produce the corresponding unsaturated carboxylic acid.
Unsaturated nitriles, such as acrylonitrile and methacrylonitrile, have been industrially produced as important intermediates for the preparation of fibers, synthetic resins, synthetic rubbers, and the like. A currently popular method for producing such nitriles is to subject an olefin, such as propene, to a vapor phase catalytic reaction with ammonia, in the presence of a suitable catalyst, at a high temperature. Again, in view of the price difference between alkanes and their corresponding alkenes, efforts have been made to develop methods and catalysts for producing acrylonitrile or methacrylonitrile from feedstocks comprising a lower alkane, such as propane or isobutane, which is reacted with ammonia in a gaseous phase, in the presence of a suitable mixed metal oxide catalyst.
For example, U.S. Patent Application Publication No. US2005/0137415 discloses a single-step vapor phase catalytic oxidation process which involves interstage condensation of a desired product from one or more product streams intermediate adjacent reaction zones arranged in series, as well as staged addition of supplemental oxygen to one or more reaction zones downstream of the first reaction zone. More particularly, the process disclosed in US 2005/0137415 is performed using a reaction system having at least two reaction zones arranged in series with one another, wherein alkanes, alkenes, and mixtures thereof, are converted to their corresponding unsaturated carboxylic acids or unsaturated nitriles in each of the reaction zones, in the presence of at least one suitable catalyst, and at least a portion of the intermediate product stream of each reaction zone is fed to the subsequent downstream reaction zone, until the final reaction zone is reached.
In the process disclosed in U.S. Patent Application Publication No. US2005/0137415, less than the full stoichiometrically required amount of oxygen is fed to the first reaction zone with the reactants, and at a least a portion of the desired product is separated (e.g., by condensation) from the intermediate product stream of at least one reaction zone, while the remaining portion of the intermediate product stream is fed to the subsequent downstream reaction zone, along with supplemental oxygen. Unexpected increases in the overall yield of desired product were achieved. The overall yield of this process unexpectedly exceeded the cumulative sum of the independent yields of each of the reaction zones.
Catalyst formulations suitable for use in the aforesaid oxidation process are proprietary to the catalyst suppliers, but the technology is well-established and such catalysts are commercially available from various sources. Various suitable catalysts, and methods of making same, are disclosed and fully described in U.S. Pat. Nos. 5,380,933, 6,383,978, 6,403,525, 6,407,031, 6,407,280, 6,461,996, 6,472,552, 6,504,053, 6,589,907 and 6,624,111. Based on recent research, it appears that catalyst formulations which contain a mixed metal oxide comprising, as essential components, molybdenum (Mo) and vanadium (V) are particularly suitable for use in commercial processes for single-step vapor phase catalytic oxidation of alkanes, alkenes, and mixtures thereof to their corresponding unsaturated carboxylic acids and unsaturated nitriles.
For example, U.S. Pat. No. 5,380,933 discloses a catalyst useful for producing an unsaturated carboxylic acid from an alkane by single-step vapor phase catalytic oxidation, where the catalyst contains a mixed metal oxide comprising, as essential components, Mo, V, Te, X and oxygen (O), wherein X is at least one element selected from the group consisting of niobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron, indium and cerium; and wherein the proportions of the respective essential components, based on the total amount of the essential components, exclusive of oxygen, satisfy the following relationships: 0.25<r(Mo)<0.98, 0.003<r(V)<0.5, 0.003<r(Te)<0.5 and 0.003<r(X)<0.5, wherein r(Mo), r(V), r(Te) and r(X) are the molar fractions of Mo, V, Te and X, respectively, based on the total amount of the essential components exclusive of oxygen. (supported?)
Similarly, U.S. Patent Application Publication No. 2005/0239643 discloses structured catalysts suitable for conversion of alkanes, alkenes, and mixtures thereof to their corresponding unsaturated carboxylic acids. These catalysts comprise one or more mixed metal oxides, each of which satisfies the expression:MoVaNbbXcZdOn wherein X is at least one element selected from the group consisting of Te and Sb, Z is at least one element selected from the group consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Si, Pb, P, Bi, Y, Ce, rare earth elements and alkaline earth elements, 0.1≦a≦1.0, 0.01≦b≦1.0, 0.01≦c≦1.0, 0≦d≦1.0 and n is determined by the oxidation states of the other elements. The aforesaid mixed metal oxide is in contact with and supported on a three dimensional form of continuous unitary structures having openings that facilitate movement of gas phase reactants and products and the three dimensional form comprises ceramic foams and ceramic monoliths selected from the group consisting of: cordierite, alumina, zirconia, silica, aluminosilicate zeolites, phosphosilicate zeolites, other zeolites and combinations thereof. The one or more mixed metal oxide catalysts are deposited on the ceramic foams and ceramic monoliths by methods selected from the group consisting of impregnation, wash coating, slurry dip-coating, chemical vapor deposition, physical vapor deposition, precipitation and combinations thereof.
U.S. Pat. No. 5,281,745 discloses a method for producing an unsaturated nitrile comprising subjecting an alkane and ammonia in the gaseous state to catalytic oxidation in the presence of a catalyst comprising a mixed metal oxide catalyst represented by the empirical formula:MoaVbTecXxOn wherein X is at least one element selected from the group consisting of niobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron and cerium and, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, x=0.01 to 1.0 and n is a number such that the total valency of the metal elements is satisfied; and wherein the catalyst has X-ray diffraction peaks at the following angles (±0.3°) of 2θ in its X-ray diffraction pattern: 22.1°, 28.2°, 36.2°, 45.2° and 50.0°. The ammoxidation catalyst of U.S. Pat. No. 5,281,745 may be used together with a conventional carrier such as silica, alumina, titania, alumnosilicate or diatomaceous earth.
In addition, a promoted ammoxidation catalyst suitable for converting alkanes to unsaturated nitriles is disclosed in U.S. Pat. No. 6,043,186 and has the formulaMo1.0VaNbbXcZeOn where X is at least one element selected from the group consisting of tellurium and antimony; Z is at least one element selected from the group consisting of ytterbium, dysprosium and erbium; E is at least one element selected from the group consisting of neodymium, samarium, lanthanum, praseodymium, europium, gadolinium, terbium, holmium, thulium, lutetium and scandium; and a, b, c, d, e, and n are, respectively, the atomic ratios of vanadium, niobium, X, Z, E and oxygen, relative to molybdenum, wherein: 0.1≦a≦1.0, 0.01≦b≦1.0, 0.01≦c≦1.0, 0≦d≦0.1, 0≦e≦0.1, 0.001≦d+e≦0.1, and n is a number determined by and consistent with the valence requirements of the other elements present in the catalyst. The catalyst may be supported on a silica carrier.
U.S. Pat. No. 6,043,185 also discloses a catalyst useful in the manufacture of acrylonitrile or methacrylonitrile by vapor phase catalytic reaction of a paraffin, selected from propane and isobutene, with molecular oxygen and ammonia, by contact of the reactants in a reaction zone with a catalyst having the empirical formula:MoaVbSbcGadXeOx where X is one or more of As, Te, Se, Nb, Ta, W, Ti, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, B, In, Ce, Re, Ir, Ge, Sn, Bi, Y, Pr, an alkali metal and an alkaline earth metal; and when a=1, b=0.0 to 0.99, c=0.01 to 0.9, d=0.01 to 0.5, e=0.0 to 1.0 and x is determined by the oxidation state of the cations present.
It is generally understood in the field of catalysis that increasing the surface area of a solid phase catalyst, such as by loading or depositing it onto a support material, may increase product yield, and that suitable supports vary depending on the nature of the starting materials and operating conditions to which the catalyst will be exposed. It is also known that reduction of the mass quantity of catalyst disposed in a particular reaction zone generally results in a proportional decrease in desired product yield.
The present invention provides a process for preparation of unsaturated carboxylic acids or unsaturated nitriles from their corresponding alkanes, alkenes, and mixtures thereof, in the presence of a supported mixed metal oxide catalyst. The process of the present invention, delivers unexpected increases in selectivity and yield which exceed the expected sum of selectivities and yields of the combined features.